1. Field of the Invention
The present invention relates to a conductive paste used for a ceramic printed circuit substrate formed of a glass ceramic and to a ceramic printed circuit substrate that uses a conductive paste as material for a printed circuit.
2. Description of the Related Art
Given recent trends toward increasingly compact and higher performance electronic apparatus, it is apparent that demands for further compactness and further improvement in electric characteristics of electronic components and printed circuit substrates will become more severe. Conventionally, a sequential lamination process and a simultaneous lamination process have been employed in order to attain compactness of electronic components and printed circuit substrates. The sequential lamination process repeats a step of printing circuits on an insulation layer and a subsequent firing step. According to the simultaneous lamination process, ceramic green sheets on which circuits are printed by use of a conductive paste are laminated and are then fired simultaneously.
The simultaneous lamination process, which involves simultaneous firing, uses a ceramic material, such as alumina or lead titanate, as an insulator. Since a firing temperature for such a ceramic material is 1000xc2x0 C. or higher, a wiring material must be a metal having a melting point not lower than 1000xc2x0 C., such as tungsten or palladium. These high melting point metals have a high electrical resistivity and thus involve a drawback in that an electric signal flowing through a conductor of such a metal suffers a large loss (so-called conductor loss). As a result, electronic components and printed circuit substrates using these materials encounter difficulty in satisfying market demands for electric characteristics.
In order to meet market demands that are becoming increasingly severe, there have been developed various kinds of electronic components and printed circuit substrates that use a low firing temperature material, which can be fired at a temperature not higher than 1000xc2x0 C., and that allow use of a conductor material having low electrical resistivity, such as silver, gold, or copper. Particularly, since silver has low electrical resistivity and can be fired in an oxidizing atmosphere, in distinction from copper, the technology of simultaneous firing of silver and a low firing temperature material has been studied and developed, thereby improving electric characteristics of ceramic printed circuit substrates.
However, silver involves problems in relation to reliability. Specifically, circuit lines of silver are apt to suffer solder erosion, and silver ions migrate and cause a short circuit between circuit lines. Accordingly, a printed circuit substrate has not been able to employ a surface circuit pattern formed of silver. Conventionally, a surface circuit pattern of silver is plated with, for example, nickel, in order to prevent solder erosion during, for example, solder mounting of a printed circuit substrate on another board or mounting of an integrated circuit chip on the printed circuit substrate. However, this plating practice involves a problem in that pretreatment by means of a chemical solution, such as acid or alkali, impairs adhesion of conductors to the substrate. Also, employment of the plating step involves a corresponding increase in cost.
Use of a silver-palladium conductor or silver-platinum conductor is effective for solving problems involved in using a silver conductor, such as solder erosion and migration of silver ions. The silver-palladium and silver-platinum conductors are obtained through addition to silver of palladium or platinum having high heat resistance. The palladium concentration of the silver-palladium conductive material is 5 to 30%, and the platinum concentration of the silver-platinum conductive material is 0.1 to 5%. These conductive materials are often used in the form of a conductive paste that undergoes postfiring metallization in a thick-film process. Postfiring in the thick-film process is performed after a conductive paste is applied to a fired substrate in a defined circuit pattern through printing and is adapted to fire the applied conductor at a temperature not higher than the substrate firing temperature.
The technology of forming a surface circuit pattern through postfiring of a thick-film process is disclosed in, for example, the following publications. Japanese Patent Application Laid-Open (kokai) No. 4-88067 discloses a conductive paste formulated through addition of an oxide of manganese, chromic oxide, and glass frit to a silver-palladium material. This disclosed conductive paste yields the effect of improving bonding strength and solder wettability of a surface circuit pattern that has undergone high-temperature aging. Japanese Patent Publication (kokoku) No. 6-50705 discloses a conductive paste formulated through addition of silicon dioxide and glass frit to silver. This disclosed conductive paste yields the effect of improving bonding strength and solder erosion resistance of a surface circuit pattern. Japanese Patent Publication (kokoku) No. 5-14363 discloses a conductive paste formulated through addition to silver of bismuth oxide, copper oxide, manganese dioxide and glass frit. This disclosed conductive paste yields the effect of improving bonding strength and solder erosion resistance of a surface circuit pattern. Since the disclosed techniques employ postfiring of a thick-film process for formation of a surface circuit pattern, a corresponding increase in labor is involved, causing an increase in fabrication cost. Since glass frit is softened during firing and stagnates between conductor particles, when the surface of a conductor layer is eroded by solder, a conductor surface having glass portions appears, with the result that the solder is repelled. Since a silver-palladium material has a relatively high electrical resistivity, a surface circuit pattern formed of the material involves a problem in that an electric signal suffers a relatively large conductor loss.
In order to reduce fabrication cost, the technique for simultaneously firing a surface circuit pattern formed of a silver-based material and a substrate has been studied. In contrast to the above-mentioned thick-film process, not many disclosed inventions employ a simultaneous firing process for forming a surface circuit pattern on a substrate. The simultaneous firing process in this context is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) Nos. 9-198919 and 9-74256 and Japanese Patent Publication (kokoku) No. 5-74166. Japanese Patent Application Laid-Open Nos. 9-198919 and 9-74256 disclose a printed circuit substrate that uses a conductive paste formulated through addition of vanadium pentoxide to silver, thereby improving bonding strength and solder wettability of a surface circuit pattern, as well as warp resistance of the substrate. Japanese Patent Publication No. 5-74166 discloses a conductive paste formulated through addition of molybdenum and/or tungsten to a noble metal, such as silver, palladium, or platinum. This disclosed conductive paste yields the effect of improving solder wettability of a surface circuit pattern.
Even when a surface circuit pattern is formed of a silver-platinum conductor in an attempt to reduce the electrical resistivity of the surface circuit pattern and to improve resistance of the surface circuit pattern to soldering heat and solder erosion, the following problems arise. (1) Initial bonding strength of a circuit conductor is low; (2) the surface circuit pattern exhibits a significant deterioration of strength in an aging test conducted at a temperature as high as 150xc2x0 C.; (3) the surface circuit pattern suffers solder erosion at a soldering heat resistance test conducted by use of a solder having a temperature as high as 260xc2x0 C.; and (4) a mismatch in firing timing between the surface circuit pattern and the corresponding substrate causes warpage of the resultant printed circuit substrate.
A conceivable cause for the above problems (1) and (2) is described below. A substrate and a silver-platinum conductor are bonded together simply by means of a glass component that has thermally diffused from the substrate to the silver-platinum conductor during firing. Such a glass bond structure breaks easily when subjected to a mechanical or thermal action. When a conductive paste that contains glass frit is used as material for a circuit conductor, an excessive amount of glass appears on the surface of a circuit conductor during firing. Therefore, the conductive paste must not contain glass frit. In other words, the conductive paste of silver-platinum must contain a certain additive that replaces glass frit and can establish such a bond structure that does not easily break even when subjected to a mechanical or thermal action.
A conceivable cause for the above problem (3) is described below. Through addition to silver of platinum having high heat resistance, the resultant silver-platinum conductor exhibits heat resistance higher than that of simple silver. Even so, the silver-platinum conductor cannot resist silver""s diffusing into solder having a temperature as high as 260xc2x0 C. Accordingly, the conductive paste of silver-platinum must contain a certain additive that yields the effect of suppressing diffusion of silver into solder.
A conceivable cause for the above problem (4) is described below. The growth of silver particles begins in a low temperature region ranging from about 200xc2x0 C. to about 300xc2x0 C., whereas the substrate is formed of glass ceramic having a softening point not lower than 500xc2x0 C. As a result, the timing of firing shrinkage of the substrate does not match that of silver. Accordingly, the conductive paste of silver-platinum must contain a certain additive that yields the effect of narrowing the gap between the timing of firing shrinkage of silver and that of the substrate.
In view of the foregoing, an object of the present invention is to provide a conductive paste that exhibits excellent performance in terms of initial bonding strength of a circuit conductor, solder wettability, deterioration upon subjection to high temperature aging, resistance to soldering heat of 260xc2x0 C. and substrate warpage and to provide a ceramic printed circuit substrate using the conductive paste.
According to a first aspect of the present invention, a conductive paste comprises 100 parts by weight of silver-platinum; 0.2 to 1 part by weight of manganese dioxide; 0.2 to 1 part by weight of copper oxide; 0.3 to 1 part by weight of silicon dioxide; and 3 to 5.6 parts by weight of molybdenum and tungsten powder.
The above conductive paste yields the following effects. (1) Addition of manganese dioxide improves the resistance of a surface circuit pattern to soldering heat of 260xc2x0 C. (2) Addition of silicon dioxide improves the aging characteristic of the surface circuit pattern as observed upon subjection to aging at a temperature as high as 150xc2x0 C. (3) Addition of copper oxide improves the initial bonding strength of the surface circuit pattern. (4) Addition of molybdenum and tungsten powder reduces warpage of a substrate. (5) Absence of glass frit improves solder wettability.
According to a second aspect of the present invention, a conductive paste comprises 100 parts by weight of silver-platinum; 0.2 to 1 part by weight of manganese dioxide; 0.2 to 1 part by weight of copper oxide; 0.3 to 1 part by weight of silicon dioxide having a specific surface area of not less than 50 m2/g as measured by a BET method, an average primary grain size of 5 to 50 nm and a purity not lower than 99.8%; and 3 to 5.6 parts by weight of molybdenum and tungsten powder.
The above conductive paste yields the following effects. (1) Addition of manganese dioxide improves the resistance of a surface circuit pattern to soldering heat of 260xc2x0 C. (2) Addition of silicon dioxide powder having the predetermined physical properties improves the aging characteristic of the surface circuit pattern as observed upon subjection to aging at a temperature as high as 150xc2x0 C. As known among those in the art, during aging at 150xc2x0 C., tin contained in solder (for example, a tin-lead eutectic solder) migrates through grain boundaries between silver and platinum and attacks a glass bond interface established by glass frit between a circuit portion and an insulation portion, thereby breaking a glass bond structure with a resultant deterioration in aging characteristic. Through addition to the conductive paste of silicon dioxide powder having the above physical properties, migration of tin is suppressed to thereby effectively improve the aging characteristic. (3) Addition of copper oxide improves the initial bonding strength of the surface circuit pattern. (4) Addition of molybdenum and tungsten powder reduces warpage of a substrate. (5) Absence of glass frit improves solder wettability.
According to a third aspect of the present invention, a ceramic printed circuit substrate comprises an insulation portion formed of glass ceramic containing lead borosilicate glass as a glass component and a circuit portion containing silver as a main component. At least part of the circuit portion is formed by use of a conductive paste comprising 100 parts by weight of silver-platinum, 0.2 to 1 part by weight of manganese dioxide, 0.2 to 1 part by weight of copper oxide, 0.3 to 1 part by weight of silicon dioxide, and 3 to 5.6 parts by weight of molybdenum and tungsten powder.
When a printed circuit is formed on the above-described green substrate (formed of glass ceramic containing lead borosilicate glass as a glass component) by use of the above-described conductive paste (comprises silver-platinum, manganese dioxide, silicon dioxide, copper oxide, and molybdenum and tungsten powder) and simultaneous firing is performed, the resultant ceramic printed circuit substrate exhibits excellent performance in terms of initial bonding strength of a circuit conductor, deterioration upon subjection to high temperature aging, resistance to soldering heat of 260xc2x0 C. and substrate warpage, and enjoys high reliability and low fabrication cost. Glass ceramic containing lead borosilicate glass as a glass component exhibits relatively stable shrinkage behavior during firing. Thus, a difference in firing shrinkage between a circuit conductor and the corresponding substrate can be easily narrowed. Particularly, glass ceramic that contains as a glass component lead borosilicate glass, which has a softening point of 650xc2x0 C. to 780xc2x0 C., is preferred.
According to a fourth aspect of the present invention, a ceramic printed circuit substrate comprises an insulation portion formed of glass ceramic containing lead borosilicate glass as a glass component and a circuit portion containing silver as a main component. At least part of the circuit portion is formed by use of a conductive paste comprising 100 parts by weight of silver-platinum; 0.2 to 1 part by weight of manganese dioxide; 0.2 to 1 part by weight of copper oxide; 0.3 to 1 part by weight of silicon dioxide having a specific surface area of not less than 50 m2/g as measured by a BET method, an average primary grain size of 5 to 50 nm and a purity not lower than 99.8%; and 3 to 5.6 parts by weight of molybdenum and tungsten powder.
When a printed circuit is formed on the above-described green substrate (formed of glass ceramic containing lead borosilicate glass as a glass component) by use of the above-described conductive paste (comprising silver-platinum, manganese dioxide, silicon dioxide powder having predetermined physical properties, copper oxide, and molybdenum and tungsten powder) and simultaneous firing is performed, the resultant ceramic printed circuit substrate exhibits excellent performance in terms of initial bonding strength of a circuit conductor, deterioration upon subjection to high temperature aging, resistance to soldering heat of 260xc2x0 C., and substrate warpage, and enjoys high reliability and low fabrication cost. Particularly, through addition to the conductive paste of silicon dioxide powder having the above physical properties, an aging characteristic as observed upon subjection to aging at a temperature as high as 150xc2x0 C. is effectively improved. Glass ceramic containing lead borosilicate glass as a glass component exhibits relatively stable shrinkage behavior during firing. Thus, a difference in firing shrinkage between a circuit conductor and the corresponding substrate can be easily narrowed. Particularly, glass ceramic that contains as a glass component lead borosilicate glass, which has a softening point of 650xc2x0 C. to 780xc2x0 C., is preferred.
The ceramic printed circuit substrates according to the third and fourth aspects are applied to, for example, electronic components, such as laminated LC filters, couplers (directional couplers), low-pass filter incorporated couplers, power distributors, baluns (balanced-to-unbalanced converters), mixer module substrates, PLL module substrates, VCOs (voltage-controlled oscillators), and TCXOs (temperature compensated crystal oscillators). The ceramic printed circuit substrates of the invention maintain a sufficiently high reliability even after subjection to a solder mounting step performed by use of a conventional eutectic solder and are sufficiently compatible with a solder mounting step performed by use of a leadless high melting point solder.
Application examples other than the above electronic components include ceramic printed circuit substrates equipped with electrode pads for electrical connection to a flip-chip bonding integrated circuit chip; specifically, so-called C4 (Controlled Collapse Chip Connection) packages and CSPs (Chip Size Packages). At least one component selected from among resistors, capacitors and inductors may be integrally mounted on these packages to form modules. The ceramic printed circuit substrates of the invention maintain sufficiently high reliability even after an integrated circuit chip is mounted thereon by use of a conventional eutectic solder and are sufficiently compatible with a step of mounting an integrated circuit chip thereon by use of a leadless high melting point solder, or a gold-tin brazing material.